The SPPV14
The elimination of infected cells through programmed cell death (apoptosis) is the most common response against infectious agents, acting as a primary defense mechanism. In order to prevail in face of such mechanisms, several viruses express inhibitors that prevent apoptosis, combating the host's innate defenses. Sheeppox virus expresses SPPV14, an inhibitor that intervenes in the pro- or anti-apoptotic pathways mediated by the Bcl-2 protein family, exhibiting a high inhibitory capacity for apoptosis in virus-infected cells. Unlike most viral strategies, which target the production of Bcl-2 homologs (anti-apoptotic)[1], SPPV14 is a potent inhibitor of Hrk, Bax and Bak (pro-apoptotic)[2]. This protein shares high similarity with DPV022 (deerpox), which is structurally the closest to SPPV14.
Structure and Function
In the crystal structures of SPPV14 bound to BH3 peptide motifs, it is observed that SPPV14 is a monomeric structure. The possibility of it being a homodimer in a cellular context cannot be excluded, but the data show that its active form is monomeric. SPPV14 is a prosurvival protein that utilizes the canonical ligand-binding grove to engage BH3 motif peptides of proapoptotic Bcl-2 proteins[2]. binds to through specific interactions, which are essential for the formation of the heterodimeric complex AB between SPPV14 and the BH3 peptide.
SPPV14 has a globular form, consisting of a surrounded by , thus forming the classical structure and fold of Bcl-2. The canonical binding site of SPPV14 is formed by the Ⲁ2-Ⲁ5 helices. The action of the SPPV14 is by sequestering BH3-only proteins including Bim, Bid, Bmf, Hrk, and Puma as well as Bak and Bax; SPPV14 strongly binds with Hrk and Bax, and the interactions are primarily mediated by the 4 hydrophobic canonical sites present in SPPV14, as well as numerous ionic interactions and hydrogen bonds. Unlike other vBcl-2, SPPV14 forms or with Bax D68, reminiscent of a highly conserved ionic bond between Arg in the BH1 region, pro-survival motif of mammals, and the Bcl-2 protein that retains the Asp of the functional BH3 motif[1].
Interactions with Bax
The interaction between SPPV14 and Bax relies on hydrophobic interactions as well as hydrogen bonds. The conserved Leu59, Leu63, Ile66, and Leu70 from Bax fit into the corresponding four hydrophobic pockets of the SPPV14-binding groove. Moreover, the M74 (in pink) occupies a fifth pocket formed by residues Cys38, Val41, Ile42, Phe133, and Asn137 (in blue) of SPPV14-binding groove, in addition to the four canonical hydrophobic residues. between the hydroxyl group of SPPV14 Ser81 and the lysyl group of Bax Lys64 are also observed, and another between the main chain carbonyl of SPPV14 Ile74 and the hydroxyl group of Bax Ser60[2].
Interactions with Hrk
For the Hrk, the interaction seen between it and SPPV14 happens on the canonical ligand-binding groove formed by helices α2-α5, which engages Hrk BH3. The three conserved hydrophobic residues L37, I40 and L44 as well as T35 from Hrk protrude into the SPPV14-binding groove and engage the resident four hydrophobic pockets. In addition to these hydrophobic interactions, SPPV14 forms an ionic interaction with Hrk via SPPV14 R84 and Hrk D42 carboxyl group. Furthermore, two hydrogen bonds are observed between SPPV14 Y46 hydroxyl and Hrk E43 carboxyl and SPPV14 and T78 hydroxyl with Hrk A34 amide. Hrk is predicted to be an intrinsically unfolded protein, and thus, the BH3 motif interactions observed with SPPV14 are likely to be recapitulated in the full context of the protein. It’s also seen the presence of a salt bridge between the SPPV14 R84-Hrk D42 that mimics other salt bridges interaction that happens in many prosurvival Bcl-2 protein:BH3 motifs.
